Why Fats And Carbs Together Trigger Craving In The Brain

There’s nothing quite as delectable as when fats and carbs come together—each is nice on its own, but when the two unite, magic (or addiction) happens. The makers of man-made foods like french fries, muffins, Doritos, and ice cream certainly have this figured out. And a new study from Yale University looks at why, neurologically speaking, the combination of carbs with fat is so heady, and why, even when we say we don't like them, we're willing to pay more for them.

The team had people look at pictures of foods that were carbohydrate-rich (pretzels), fatty (cheese), or contained both (pastries). They had people gauge each food’s energy density/calorie content and say how appealing each was; they also stated how much they’d be willing to pay for each.

Participants were pretty good at estimating the energy density of fatty foods—but they were not very good at estimating how many calories foods that were high in both fats and carbs contained. This may help explain why it’s hard to know when to stop eating them.

People were also, not surprisingly, more willing to pay extra for the foods containing both fats and carbs. And there was a gestalt effect: The amount they’d pay for the combination foods was more than you would predict by summing the prices for fats and carbs separately. Interestingly, the price they’d pay for fat-and-carbs wasn’t linked to how much the person said they liked the foods, which again suggests that fats and carbs together tap into something beyond either one alone.

To get at this, the team also looked at what’s going on in the brain when people are viewing and bidding on the different types of foods. The responses of two brain areas in particular, the caudate and putamen, were much stronger in response to both fats and carbs together than either one alone. They also corresponded more closely with the prices people were willing to pay for fat and carbs together, compared to either of the two food types alone. These areas are intimately involved in craving, reward, and goal-directed actions, and release dopamine in response to delectable stimuli.

“Our study shows that when the signals are combined they make foods more reinforcing,” said Dana Small in a statement. She believes that the brain processes carbs and fats separately, and when the two nutrients are present in a single food, both systems are activated. Which may make us go a little crazy. Foods that are high in both fats and carbs simply aren’t found in nature—for instance, fruits are high in sugar but have no fat, whereas red meat may be high in fat but has no carbs. The only food that has both is breast milk, and it’s easy to see why a strong desire for it would be evolutionarily advantageous to an infant.

So people whose job it is to design high-arousal processed foods, like chips and donuts, are tapping into our brain’s weakness for fats and carbs in combination. And, of course, they’re likely contributing to the obesity epidemic, since these foods often trigger behaviors that aren’t so different from addiction.

“In the modern food environment that is rife with processed foods high in fat and carbohydrate like donuts, French fries, chocolate bars, and potato chips,” said Small, “this reward potentiation may backfire to promote overeating and obesity.”

But perhaps knowing just how susceptible our brains are to the combination may make us more resistant to buying the foods in the first place. Because, let's admit it, after we buy them, all bets are off.

I fell into writing about health shortly after grad school, where I realized I didn't want to work in a lab for the rest of my life! My main areas of interest are the brain and behavior, as well as what influences the decisions we make about our health, and how we can chang...